Electronic organs have heretofore employed a device called a sequencer which is used to create and control a number of repeated musical events. One function of the sequencer is to produce an effect known in classical music as "ostinati" or ground basses. For example, sequencers have been used to create rather elaborate accompaniments. While the sequencer produces the accompaniment, a musician can play the lead line on the same or another keyboard, or even another instrument. While analog sequencers have been used, digital sequencers are currently more widely accepted because they offer greater flexibility.
Known digital sequencers utilize a Read/Write memory storing a plurality of words, each word being coded to represent a note played on a keyboard. Typically, each word corresponds to one beat or a half beat. If the time resolution is an eighth note, for example, the Read/Write memory will store eight words per measure, the capacity of the memory fixing the number of measures of the music that can be played. If a note is being held as a quarter note, then two successive words must be coded for the same note. Rests are coded by having an appropriate number of blank words. Once the memory has been coded, the sequencer can be used to play the keyboard instrument by reading back the data words in the memory in time sequence. The tempo can be varied by varying the rate at which the words are read out of the memory.
Known sequencers, however, are monophonic, that is they are capable of generating or playing only one note at a time. They also do not make efficient use of memory space since they require separate coded words for each beat or half-beat of a measure including rests.